This invention relates to modified polyurethane adhesive compositions having improved viscosity and bond strength characteristics. This invention also relates to a process for laminating substrates using the polyurethane compositions herein.
Polyurethane adhesives, because of their excellent adhesive properties, are particularly desirable for use in bonding substrates. The most widely used adhesives for preparing flexible packaging laminations are based on curable polyether and polyester urethanes of low molecular weight, which are supplied as one-part and two-part flammable solvent-borne systems, both of which cure by reaction of isocyanate (NCO) groups. Other solvent-borne adhesives are fully reacted polyurethanes of high molecular weight. Water-based urethane adhesives are also known in the art.
As an alternative to these adhesives, the 100%-reactive, solvent-free adhesive system may be used, which is of two distinct types: (a) the reactive liquid adhesive, which is applied at room temperature and cured by radiation or through chemical combination, and (b) the reactive hot-melt adhesive, a solid at room temperature which melts to a highly viscous liquid when heated to moderate temperatures (82.degree.-121.degree. C.), and is applied in the molten state. The hot-melt adhesive cures on contact with ambient moisture.
The reactive liquid urethane adhesives of type (a) may be composed of one or two parts. Unsatisfactory initial bond strength and short pot life after mixing are two disadvantages of the two-part adhesives. The reactive one-part adhesive is the most convenient of the reactive liquid adhesives, yet because of the high viscosity of such adhesives, a solvent or reactive diluent is normally added thereto prior to application. In addition, residual monomers which remain after curing radiation-curable reactive liquid adhesives may produce odors or undesirable extractables.
There are several hot-melt urethane adhesives of type (b) known in the art. These urethane-based systems are superior to conventional hot-melt adhesives, which latter adhesives depend on their high molecular weight for their cohesive properties and hence have high viscosities of from 5000 to 10,000 cps. at temperatures of 150.degree.-190.degree. C. Furthermore, the conventional hot-melt compositions generally contain a variety of ingredients and are plagued by stability problems because of the high temperatures involved in their use. Finally, because these compositions are not curing, they are thermoplastic and lack resistance to solvents and heat.
The typical hot-melt urethane adhesives of the prior art contain a polyurethane prepolymer of low viscosity (such as is exemplified in Belgian Pat. No. 835,022), to which are added tackifiers, copolymers, thermoplastic polymers, adhesion promoters, etc., in varying mixtures, to enhance the initial bond strength and overall performance of the prepolymer (see, in particular, U.S. Pat. Nos. 3,931,077 and 3,991,025). Usually a combination of such additives is necessary to achieve sufficient bond strength in adhesives of this type, resulting in complicated formulations.
Certain plasticizers such as diisooctyl adipate reduce the viscosity of the hot-melt adhesive at both elevated and room temperatures. While the reduction in viscosity at higher temperatures is highly desirable, the concomitant significant reduction in viscosity at room temperature is unacceptable for applications requiring the adhesive to have initial cohesive strength. Other viscosity-modifying compounds such as terpene resins, aliphatic hydrocarbon resins, aromatic hydrocarbon resins, rosin alcohol phthalate esters, zinc salt rosin and epoxy resins are generally incompatible with urethane prepolymers and are thus of little or no use. Rosin esters of low molecular weight alcohols such as ethylene glycol and pentaerythritol, while compatible in many cases, tend to increase the viscosity of urethane prepolymers at high temperatures while not significantly affecting the viscosity at room temperature. The residual acid groups found in commercially available rosin esters also engage in undesirable reactions with the urethane prepolymer, and the presence of the rosin ester adversely affects the cohesive strength of the urethane prepolymer.
A significant improvement in the modification of urethane prepolymers is represented by U.S. Pat. No. 4,242,488 issued Dec. 30, 1980 to Stanley et al. In this patent the additive employed in conjunction with the prepolymer is a monomeric carbamate ester which significantly decreases the viscosity of urethane prepolymers at higher temperature while only minimally decreasing or in some cases increasing the room temperature viscosity and cohesive strength of the prepolymer. However, the carbamate esters used therein are chemically non-reactive compounds, and although they enhance the uncured physical properties of the urethane prepolymers by physical means, they remain as an inert filler in the cured urethane prepolymer and detract from the cured urethane prepolymer in proportion to the amount used to modify the urethane prepolymer. As long as the performance requirements on the modified urethane prepolymer are modest, such as in the lamination of films used in flexible packaging, the decrease in performance of the cured prepolymer is fully acceptable. However, in more demanding applications such as the bonding of vinyl to particle board, and the lamination of automotive headliners and seat cushions, there is a need for a modified prepolymer having better curing performance.
Accordingly, it is an object of the present invention to provide a modified polyurethane adhesive composition of simple formulation having improved bond strength, stability and curing characteristics.
It is another object to provide a process for laminating substrates and to provide a laminated film assembly wherein the modified urethane composition described herein is employed as the adhesive.